U.S. patent number 6,795,232 [Application Number 10/108,996] was granted by the patent office on 2004-09-21 for wavelength converter.
This patent grant is currently assigned to Nippon Telegraph and Telephone Corporation. Invention is credited to Makoto Abe, Koji Enbutsu, Kazuo Fujiura, Tadayuki Imai, Eishi Kubota, Takashi Kurihara, Masahiro Sasaura, Seiji Toyoda, Shogo Yagi.
United States Patent |
6,795,232 |
Fujiura , et al. |
September 21, 2004 |
Wavelength converter
Abstract
A wavelength converter implements high speed, high efficiency,
low noise wavelength conversion without performing high voltage
poling of a crystal, and enables switching and modulation of
converted light in response to an electric field. A KLTN crystal,
includes a deposited-gold electrode within its incidence plane, and
is connected to a DC power supply via a copper wire. The crystal
material is composed of KTa.sub.1-x Nb.sub.x O.sub.3 and/or
K.sub.1-y Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3. A polarizer
controls the polarization of the fundamental wave in the direction
parallel to the electric field, and launches it into the electrode
of the KLTN crystal. The KLTN crystal, rotating on an axis in the
direction of the electric field, launches only part of the
generated SHG light with the same polarization direction as that of
the incident light into a photo multiplier tube through a
polarizer.
Inventors: |
Fujiura; Kazuo (Ibaraki,
JP), Yagi; Shogo (Ibaraki, JP), Imai;
Tadayuki (Ibaraki, JP), Enbutsu; Koji (Ibaraki,
JP), Sasaura; Masahiro (Ibaraki, JP),
Kurihara; Takashi (Ibaraki, JP), Abe; Makoto
(Ibaraki, JP), Toyoda; Seiji (Ibaraki, JP),
Kubota; Eishi (Ibaraki, JP) |
Assignee: |
Nippon Telegraph and Telephone
Corporation (Tokyo, JP)
|
Family
ID: |
26612969 |
Appl.
No.: |
10/108,996 |
Filed: |
March 29, 2002 |
Foreign Application Priority Data
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Apr 2, 2001 [JP] |
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2001-103552 |
Apr 3, 2001 [JP] |
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2001-104943 |
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Current U.S.
Class: |
359/326; 359/330;
385/8; 385/2; 385/122; 359/332 |
Current CPC
Class: |
G02F
1/3534 (20130101); G02F 1/3551 (20130101); G02F
1/3548 (20210101) |
Current International
Class: |
G02F
1/355 (20060101); G02F 1/35 (20060101); G02F
001/35 () |
Field of
Search: |
;385/122 ;359/326-332
;372/21,22 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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06110095 |
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Apr 1994 |
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JP |
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08160476 |
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Jun 1996 |
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JP |
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Primary Examiner: Lee; John D.
Assistant Examiner: Knauss; Scott Alan
Attorney, Agent or Firm: Venable LLP Sartori; Michael A.
Vivarelli; Daniel G.
Claims
What is claimed is:
1. A wavelength converter for producing converted light with a
wavelength corresponding to an energy difference between signal
light and pumping light with a wavelength different from that of
the signal light, by launching the signal light and the pumping
light into a crystal material simultaneously, wherein said crystal
material consists of a crystal composed of at least one of
KTa.sub.1-x Nb.sub.x O.sub.3 and K.sub.1-y Li.sub.y Ta.sub.1-x
Nb.sub.x O.sub.3 wherein, when an electric field is applied to the
crystal material, converted light is produced, and when the
electric field is removed, substantially converted light is
produced.
2. The wavelength converter as claimed in claim 1, wherein said
crystal material comprises at least one comb electrode with an
electrode period that will establish quasi-phase matching between
the signal light and the pumping light.
3. The wavelength converter as claimed in claim 2, wherein said at
least one electrode comprises at least two electrodes with
different periods.
4. The wavelength converter as claimed in claim 2, wherein said
crystal material comprises an electrode structure that enables the
electric field to be applied to said electrode structure in at
least two directions.
5. The wavelength converter as claimed in claim 2, wherein said
crystal material comprises a core with a high refractive index and
a cladding with a low refractive index, both of them being composed
of at least one of KTa.sub.1-x Nb.sub.x O.sub.3 and K.sub.1-y
Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3 with different
compositions.
6. The wavelength converter as claimed in claim 2, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
7. The wavelength converter as claimed in claim 1, wherein said
crystal material comprises an electrode structure that enables the
electric field to be applied to said electrode structure in at
least two directions.
8. The wavelength converter as claimed in claim 7, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
9. The wavelength converter as claimed in claim 1, wherein said
crystal material comprises a core with a high refractive index and
a cladding with a low refractive index, both of them being composed
of at least one of KTa.sub.1-x Nb.sub.x O.sub.3 and K.sub.1-y
Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3 with different
compositions.
10. The wavelength converter as claimed in claim 9, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
11. The wavelength converter as claimed in claim 1, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
12. The wavelength converter as claimed in claim 1, wherein the
converted light is modulated by modulating the electric field
applied to said crystal material.
13. The wavelength converter as claimed in claim 12, wherein said
crystal material comprises at least one comb electrode with an
electrode period that will establish quasi-phase matching between
the signal light and the pumping light.
14. The wavelength converter as claimed in claim 13, wherein said
at least one electrode comprises at least two electrodes with
different periods.
15. The wavelength converter as claimed in claim 12, wherein said
crystal material comprises an electrode structure that enables the
electric field to be applied to said electrode structure in at
least two directions.
16. The wavelength converter as claimed in claim 12, wherein said
crystal material comprises a core with a high refractive index and
a cladding with a low refractive index, both of them being composed
of at least one of KTa.sub.1-x Nb.sub.x O.sub.3 and K.sub.1-y
Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3 with different
compositions.
17. The wavelength converter as claimed in claim 12, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
18. The wavelength converter as claimed in claim 1, further
comprising at least two electrodes with different periods.
19. The wavelength converter as claimed in claim 18, wherein said
crystal material comprises a core with a high refractive index and
a cladding with a low refractive index, both of them being composed
of at least one of KTa.sub.1-x Nb.sub.x O.sub.3 and K.sub.1-y
Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3 with different
compositions.
20. The wavelength converter as claimed in claim 18, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
21. The wavelength converter as claimed in claim 1, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
22. A wavelength converter operating as a multi-wavelength light
source including a planar optical waveguide comprising: a core with
a high refractive index composed of a crystal material with a
composition of at least one of KTa.sub.1-x Nb.sub.x O.sub.3 and
K.sub.1-y Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3 ; a cladding
surrounding said core; at least one electrode that is formed on a
surface of said optical waveguide and has a fixed electrode period;
signal light generating means for generating signal light with at
least one wavelength; and pumping light generating means for
generating pumping light with a wavelength different from that of
the signal light output from said signal light generating means,
wherein the signal light and the pumping light are launched
simultaneously into said optical waveguide to generate converted
light with at least one wavelength wherein, when an electric field
is applied to the crystal material, converted light is produced,
and when the electric field is removed, substantially converted
light is produced.
23. The wavelength converter as claimed in claim 22, wherein the
electrode period satisfies quasi-phase matching condition required
for differential frequency generation based on an energy difference
between the signal light and the pumping light.
24. The wavelength converter as claimed in claim 23, wherein said
electrode has a structure that enables the electric field to be
applied in at least one of two directions parallel to a direction
of an electric field of TE polarization of the signal light and
parallel to a direction of TM polarization of the signal light.
25. The wavelength converter as claimed in claim 23, wherein the
converted light is modulated by modulating a voltage applied to
said electrode.
26. The wavelength converter as claimed in claim 23, wherein said
at least one electrode comprises a plurality of electrodes with
different periods.
27. The wavelength converter as claimed in claim 23, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
28. The wavelength converter as claimed in claim 22, wherein said
electrode has a structure that enables the electric field to be
applied in at least one of two directions parallel to a direction
of an electric field of TE polarization of the signal light and
parallel to a direction of TM polarization of the signal light.
29. The wavelength converter as claimed in claim 28, wherein the
converted light is modulated by modulating a voltage applied to
said electrode.
30. The wavelength converter as claimed in claim 28, wherein said
at least one electrode comprises a plurality of electrodes with
different periods.
31. The wavelength converter as claimed in claim 28, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
32. The wavelength converter as claimed in claim 22, wherein the
converted light is modulated by modulating a voltage applied to
said electrode.
33. The wavelength converter as claimed in claim 32, wherein said
at least one electrode comprises a plurality of electrodes with
different periods.
34. The wavelength converter as claimed in claim 32, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
35. The wavelength converter as claimed in claim 22, wherein said
at least one electrode comprises a plurality of electrodes with
different periods.
36. The wavelength converter as claimed in claim 35, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
37. The wavelength converter as claimed in claim 22, further
comprising an application halting means for halting applying the
electric field when a temperature of a waveguide is dropped below a
phase transition temperature with applying the electric field,
wherein said wavelength converter carries out wavelength conversion
after said application halting means halts applying the electric
field.
Description
This application is based on Japanese Patent Application Nos.
2001-103552 filed Apr. 2, 2001 and 2001-104943 filed Apr. 3, 2001,
the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength converter used for
optical communication, optical measurement or display devices, and
more particularly to a wavelength converter applicable to optical
signal processing that requires high speed, high efficiency and low
noise wavelength conversion. In addition, the present invention
relates to a wavelength converter as a multi-wavelength light
source used for wavelength division multiplexing communication
requiring low noise signal light with multiple wavelengths and
accurate channel spacing.
2. Description of the Related Art
Conventionally, a wavelength tunable laser, which is implemented by
irradiating a crystal or a liquid or gas medium, which possesses
second order or third order nonlinearity, with a high power laser
beam to convert the laser beam to a wavelength region the laser
cannot oscillate, is applicable as a wide range wavelength tunable
light source. This technique is generally called an optical
wavelength conversion using nonlinear optical media. As for
materials of the wavelength conversion media utilizing the
secondary nonlinear optical effect, inorganic crystals are applied
to many wavelength conversion media at present.
To implement such wavelength conversion, an optical waveguide is
often employed to make effective use of the nonlinear optical
coefficient of the material. The wavelength converters proposed so
far include those utilizing the cross gain modulation, cross phase
modulation, and four wave mixing (optical mixing using third-order
nonlinear polarization) of optical semiconductors.
In addition, the phase matching is considered as an effective
method to be applied to inorganic materials such as KTP and
LiNbO.sub.3, and techniques are proposed which utilize temperature
tuning, angle tuning, and quasi-phase matching in which less
cancellation takes place between a nonlinear polarization wave
based on a fundamental wave and a propagation high frequency
generated.
As for the wavelength conversion utilizing optical semiconductors
that are under development at present, they are inapplicable to
optical communication or optical measurement that requires high
speed and low noise because they have large noise due to their
spontaneous emission light, and their speed limit due to carrier
lifetime. In addition, although LiNbO.sub.3 quasi-phase matching
devices are proposed as a high-speed, low-noise wavelength
converter, they have drawbacks such as insufficient conversion
efficiency, requiring an interaction length of at least 5 cm to
achieve preferable conversion efficiency. Furthermore, it has a
problem of having polarization sensitivity that the conversion
efficiency varies sharply depending on the orientation of the
crystal.
Moreover, the domain inversion for the quasi-phase matching must
undergo poling using a high voltage, offering a problem of low
yields. Besides, since the domain inversion by the poling must be
formed such that it makes phase matching with a specified
wavelength, the wavelength of the pumping light must be fixed.
As a result, the wavelength converter fabricated has a problem in
that it can convert only to a fixed wavelength, and hence cannot
convert to a wavelength required. The converting function to a
desired wavelength is needed for equipment such as optical
switching systems and optical routers, which carry out routing
using wavelengths as routing information. In addition, the function
is important to circumvent blocking of wavelengths, which can occur
when multiple wavelength signals are supplied to a single
system.
At present, installation of wavelength division multiplexing (WDM)
systems is accelerated to implement large capacity communications.
The WDM systems can reduce the cost of a system by transmitting
multiple signals with different wavelengths through a single
optical fiber. Therefore, it can increase the transmission capacity
without installing a new fiber.
Although the method has an advantage in the fiber installation
cost, it has a problem of requiring many light sources with high
wavelength accuracy to achieve high density. Up to now, a method is
used which selects semiconductor lasers that precisely fit to the
wavelengths of the signal light, and disposes them by the number
required. This method, however, has a problem of increasing cost
because of the selection of lasers suitable for the
wavelengths.
Alternatively, a method using a semiconductor mode-locking laser or
fiber ring laser is also proposed. In addition, a spectral slice
light source is proposed which slices supercontinuum (SC) light
that is generated by the short-pulse light source and nonlinear
optical fiber by an arrayed waveguide grating demultiplexer.
However, since it requires a long nonlinear fiber to generate the
SC light, it has a problem of making it difficult to reduce its
size.
SUMMARY OF THE INVENTION
The present invention is implemented considering the foregoing
problems. Therefore, an object of the present invention is to
provide a high efficiency, low noise wavelength converter that can
be implemented without the high voltage poling of a crystal, and
that can carry out switching and modulation of converted light by
using electric field.
Another object of the present invention is to provide a wavelength
converter functioning as a multi-wavelength light source capable of
controlling a wavelength band or the number of wavelengths by
selecting electrodes to which electric fields are applied.
To accomplish the objects, according to the present invention,
there is provided a wavelength converter for producing converted
light with a wavelength corresponding to an energy difference
between signal light and pumping light with a wavelength different
from that of the signal light, by launching the signal light and
the pumping light into a crystal material simultaneously, wherein
the crystal material consists of a crystal composed of at least one
of KTa.sub.1-x Nb.sub.x O.sub.3 and K.sub.1-y Li.sub.y Ta.sub.1-x
Nb.sub.x O.sub.3.
In addition, to accomplish the objects, according to a present
invention, there is provided a wavelength converter operating as a
multi-wavelength light source including a planar optical waveguide
comprising: a core with a high refractive index composed of a
crystal material with a composition of at least one of KTa.sub.1-x
Nb.sub.x O.sub.3 and K.sub.1-y Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3
; a cladding surrounding the core; an electrode that is formed on a
surface of the optical waveguide and has a fixed electrode period;
signal light generating means for generating signal light with at
least one wavelength; and pumping light generating means for
generating pumping light with a wavelength different from that of
the signal light output from the signal light generating means,
wherein the signal light and the pumping light are launched
simultaneously into the optical waveguide to generate signal light
with at least one wavelength.
Thus, the present invention is characterized in that it utilizes
the crystal with the composition of KTa.sub.1-x Nb.sub.x O.sub.3
and/or K.sub.1-y Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3 as a medium
for achieving the wavelength conversion. These KTN and KLTN
crystals are a cubic system with centrosymmetry in an applied
temperature range. Although they have no second order nonlinear
effect, they are characterized by exhibiting secondary nonlinear
effect in response to an electric field applied. Therefore, it is
possible to implement the multiple wavelength generation based on
the differential frequency generation by applying the electric
field to the electrode with the period that makes phase matching
with the signal light and pumping light.
The efficiency of the nonlinear optical effect increases in
proportion to the electric field applied, and offers a twice or
more efficiency as compared with the conventional LiNbO.sub.3
nonlinear optical crystal within a range of a practical electric
field to be applied. Accordingly, it can implement the wavelength
conversion with four times or more efficiency using the same
interaction length as the conventional LN wavelength converter, or
with the same efficiency using less than half the interaction
length. In addition, when the electric field is removed, the KTN
and KLTN crystals are simply a transparent medium without causing
any changes in the signal light. Thus, they can achieve such a
function as turning the converted light on and off by switching the
electric field on and off. In addition, the converted light can be
modulated by modulating the electric field applied.
In addition, as for the conventional wavelength converter, since
the LN crystal is a trigonal system, the c axis must be aligned to
the polarization of the incident light to obtain the maximum
nonlinear effect, and the quasi-phase matching is achieved by
inverting the spontaneous polarization in the c axis direction.
Therefore, in the differential frequency generation by the LN
wavelength converter, the polarization direction of convertible
light is limited by the direction of the domain inversion produced,
making it impossible to achieve high conversion efficiency in the
other polarization. In contrast, the KTN and KLTN used in the
present invention are an isotropic crystal with exhibiting a
nonlinear characteristic in the direction of the applied electric
field. Thus, they have an advantage of being able to implement the
polarization insensitive wavelength converter easily with such a
structure as including two electrodes perpendicular to each other
to which the electric fields are applied.
Furthermore, the wavelength converter in accordance with the
present invention has an advantage of being able to obviate the
need for the high voltage poling of the crystal which is required
by the conventional LN wavelength converter, and to implement the
quasi-phase matching easily by forming the electrode. This is
because forming several type of electrodes with different periods
on the surface of the crystal makes it possible to select the
wavelength of the pumping light in accordance with the period,
thereby being able to provide the wavelength converter with those
functions. Furthermore, since the principle of the wavelength
conversion in accordance with the present invention is based on the
differential frequency generation, which is a parametric process,
it offers an advantage of high speed beyond THz and noise free
characteristic. Thus, it can implement the performance that no
wavelength conversion using the optical semiconductors can achieve.
Besides, since the converted light is generated by the interaction
between the signal light and pumping light, it is shaped up into a
pulse train consisting of short-width pulses. Accordingly, when the
pumping light consists of a short-width pulse train such as that of
a fiber-ring laser, even if the signal light is generated by a
broad light source such as a semiconductor laser generating light
including jitters, the wavelength converter in accordance with the
present invention can generate high quality light.
Furthermore, differential frequencies, the number of which
corresponds to the number of the electrodes, can be obtained by
disposing the electrodes with different periods in the direction of
the waveguide, by launching the pumping light that phase matches
with the periods, and by applying the electric fields to all the
electrodes. When the initial incident signal light has multiple
wavelengths, the number of wavelengths the device can produce is
equal to n.times.2.sup.m, where n is the number of wavelengths of
the initial incident signal light, and m is the number of
electrodes. For example, when the number of the wavelengths of the
initial incident light is 10, and the number of the electrodes is
four, it can generate 160 waves.
In addition, since the channel spacing of the signal light
generated by this method is determined by energy difference between
the channel spacing of the initial incident signal light and the
wavelength corresponding to half the energy of the pumping light,
the wavelength converter in accordance with the present invention
can generate the light with a uniform channel spacing precisely
matching the ITU-T grid.
Furthermore, it has an advantage of being able to offer high speed
beyond THz and noise free characteristic in principle. In addition,
it operates as a wavelength tunable light source by sequentially
applying the electric field via the electrodes that have
quasi-phase matching with different wavelengths. The light source
can also operate as a variable wavelength light source
incorporating a modulator, because it can generate a modulated
signal by modulating the electric field by some other method.
Although the embodiments below utilize a rectangular buried
waveguide, similar characteristics can be achieved by a diffusion
waveguide fabricated using ion diffusion.
Thus, according to the present invention, the crystal material
consists of a crystal composed of KTa.sub.1-x Nb.sub.x O.sub.3
and/or K.sub.1-y Li.sub.y Ta.sub.1-x Nb.sub.x O.sub.3 in the
wavelength converter that produces converted light with a
wavelength corresponding to an energy difference between the signal
light and pumping light with a wavelength different from that of
the signal light by launching the signal light and the pumping
light into the crystal material simultaneously. As a result, the
present invention can implement the high efficiency, low noise
wavelength conversion without performing the high voltage poling of
the crystal which is essential for the conventional wavelength
converter. In addition, it can achieve the switching and modulation
of the converted light by the electric field.
Moreover, it can achieve the polarization insensitive wavelength
conversion, which is impossible for the conventional converter.
This enables the optical signal processing indispensable for the
optical routing applied to the optical communication field, thereby
implementing a router with simple configuration at low cost. The
wavelength conversion is free from noise, and causes no signal
degradation even through the wavelength conversion is repeated by a
number of stages. Accordingly it is applicable to a router that
repeats the signal processing many times. In addition, in the
optical measurement field, it can demultiplex a ultra-fast optical
signal at high efficiency, offering an advantage of being able to
fabricate ultra-fast optical signal measuring instruments with a
simple configuration.
As for other applications, using the wavelength converter in
accordance with the present invention can implement high wavelength
conversion efficiency that cannot be achieved by conventional
devices, and the second harmonic generation by the converter makes
it possible to use it as a blue color emitted laser light
source.
Furthermore, according to the present invention, the wavelength
converter includes the electrode that is formed on a surface of the
optical waveguide and has a fixed electrode period; a signal light
generating means for generating signal light with at least one
wavelength; and a pumping light generating means for generating
pumping light with a wavelength different from that of the signal
light output from the signal light generating means, wherein the
signal light and the pumping light are launched simultaneously into
the optical waveguide to generate signal light with at least one
wavelength. Thus, it can implement a multi-wavelength light source,
which cannot be realized by the conventional technique, on a single
chip. In addition, it can control the number of wavelengths and
wavelength band by selecting the electrodes to which the electric
field is applied. Furthermore, it offers an advantage of being able
to generate the short-pulse signal light with ease. Thus, the
present invention can implement the multi-wavelength light source
applied to the wavelength division multiplexing communication with
a simple and inexpensive configuration.
As described above, the KTN crystal and KLTN crystal used in the
present invention assume that they are used as a cubic system.
However, the ferroelectric phase transition temperature from the
cubic to tetragonal system is controllable in a range of
-250.degree. C.-400.degree. C. by varying composition of the Nb and
Ta. In this case, by using a crystal with the phase transition
temperature above the room temperature, and by cooling it below the
phase transition temperature with applying the electric field via
the electrode, the spontaneous polarization occurs in the direction
of the electric field applied, and is fixed. A wavelength converter
requiring no application of the electric field can be configured by
controlling the phase transition temperature. The polarization
structure thus configured can be eliminated by elevating its
temperature beyond the phase transition temperature.
The above and other objects, effects, features and advantages of
the present invention will become more apparent from the following
description of embodiments thereof taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a configuration of a device used
for second harmonic generation in an embodiment 1 in accordance
with the present invention;
FIG. 2 is a graph illustrating a generating example of the second
harmonic in the embodiment 1;
FIG. 3 is a graph illustrating a second harmonic produced by
turning applied electric field on and off in the embodiment 1;
FIG. 4 is a perspective view showing a structure of a wavelength
converter fabricated in an embodiment 2 in accordance with the
present invention;
FIG. 5 is a graph illustrating a spectrum after the wavelength
conversion in the embodiment 2;
FIG. 6 is a perspective view showing a structure of a wavelength
converter fabricated in an embodiment 3 in accordance with the
present invention;
FIG. 7 is a perspective view showing a structure of a wavelength
converter fabricated in embodiments 4 and 6 in accordance with the
present invention;
FIG. 8 is a graph illustrating spectra in the embodiments 4 and
6;
FIGS. 9A and 9B are cross-sectional views of electrodes
perpendicular to the waveguide, wherein FIG. 9A shows an electrode
for TM polarization, and FIG. 9B shows an electrode for TE
polarization; and
FIG. 10 is a table showing wavelengths generated by applying
electric fields to electrodes at a desired time.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The embodiments in accordance with the present invention will now
be described with reference to accompanying drawings.
[Embodiment 1]
In the present embodiment 1, the wavelength conversion performance
of the KLTN crystal material is confirmed by generating a second
harmonic using a KLTN crystal material.
FIG. 1 is a diagram showing a configuration for the second harmonic
generation using a KLTN crystal. The KLTN crystal 4 is a 0.5 mm
thick plate with its both surfaces optically polished. It includes
electrodes formed within an incidence plane by depositing gold, and
connected to a DC power supply through copper wires connected to
the electrodes. The crystal material is composed of KTa.sub.1-x
Nb.sub.x O.sub.3 and/or K.sub.1-y Li.sub.y Ta.sub.1-x Nb.sub.x
O.sub.3.
A fundamental wave generator 1 generates a 1.55 .mu.m fundamental
wave in accordance with to a differential frequency between an
Nd:YAG Q-switched laser and an excimer laser. A polarizer 2
controls the polarization of the fundamental wave in the direction
parallel to the electric field, and launches it between the
electrodes of a KLTN crystal 4 mounted on a rotary stage 5 via a
lens 3. The KLTN crystal 4, rotating on an axis in the direction of
the electric field, causes the SHG (Second Harmonic Generation)
light to pass through a polarizer 7 via a lens 6. The SHG light
passes through a filter 8 so that only the light with the same
polarization direction as that of the incident light is launched
into a photo multiplier tube 8.
The incident angle dependence of the generated SHG light was
measured by this method. The SHG light was measured in the same
setup by launching the fundamental wave in the direction of the
z-axis of an LN with X-cut, and it was compared with the SHG light
intensity of the KLTN. FIG. 2 illustrates its result. In FIG. 2, a
denotes the second harmonic from the KLTN crystal of the present
embodiment 1, and b denotes the second harmonic from the LN used as
a standard sample. FIG. 2 clearly shows that the KLTN crystal
supplied with the electric field generates the SHG light a, and
that the crystal comes to have the wavelength conversion function
by the electric field applied thereto.
The angle dependence of the SHG light intensity illustrated in FIG.
2 is based on the relationship between the nonlinear coherence
length of the crystal and the interaction length of the fundamental
wave, the peak interval of which allows to estimate the depth of
the effective electric field formed in the KLTN. In this case, it
is estimated about 0.2 mm. The SHG intensity obtained by applying
the electric field of one KV/cm is about 10 times greater than that
of the LN, corresponding to about 79 pm/V in terms of a second
order nonlinear coefficient. This is the greatest second order
nonlinear coefficient among the nonlinear optical crystals reported
up to now.
Furthermore, a new electrode was formed on the surface, on which
the electrodes used in the foregoing measurement were not formed,
such that the two electrodes become perpendicular. The two
electrodes were supplied with the one KV/cm electric field, and the
fundamental wave was normally launched onto the two electrodes. In
this case, the SHG light with the same intensity was observed for
the two polarized waves, thereby demonstrating that the KLTN
crystal can achieve the wavelength conversion of the signal light
including any polarized wave by controlling the application
direction of the electric fields.
FIG. 3 illustrates the rate of change of the SHG light intensity
with respect to time, which was measured by turning the electric
field on and off. It is clear from FIG. 3 that the SHG light was
generated by applying the electric field, and eliminated by
removing it. Thus, it is obvious that the KLTN crystal functions as
a transparent medium without the electric field, and as a switch
capable of turning on and off the converted light by the on-off of
the electric field. In addition, since the nonlinear constant
varies in proportion to the applied voltage, it is clear that the
KLTN crystal operates not only as the switch, but also as a
modulator capable of modulating the converted light intensity by
the electric field.
[Embodiment 2]
A rectangular waveguide structure as shown in FIG. 4 was fabricated
using photolithography and liquid phase epitaxial technique. The
fabricated KLTN waveguide 13 has a refractive index difference of
2.5%, and the cutoff wavelength in a high-order mode is 0.6 .mu.m.
Thus, it functions as a single-mode waveguide for a long
wavelength. The fabricated waveguide was 3 cm long, and the loss of
the waveguide was 0.15 dB/cm.
FIG. 4 shows a wavelength converter fabricated in this way. A
substrate 15 was composed of SrTiO.sub.3 doped with La, and gold
was deposited on a top electrode 14. The period of the electrode
elements corresponds to the grating period that enables the
quasi-phase matching needed for the wavelength conversion of 1.55
.mu.m band using the pumping light of 0.775 .mu.m. In this case,
the period of electrode elements becomes 12 .mu.m. The outgoing
light was measured using an optical spectrum analyzer with applying
the voltage of one KV/cm to the electrode, and launching the 1.54
.mu.m signal light and 0.775 .mu.m pumping light simultaneously
into the incidence edge using a polarization-maintaining optical
fiber.
FIG. 5 is a graph illustrating a spectrum after the wavelength
conversion. In FIG. 5, the reference symbol c designates input
signal light, d designates the second-order diffraction light of
the pumping light and e designates the converted light. FIG. 5
clearly shows that the wavelength conversion is implemented by the
differential frequency generation. In addition, the signal light
and converted light undergo the parametric amplification, and the
gain of the converted light with respect to the input signal light
reaches about 15 dB, which is such a high gain that the
conventional LN wavelength converter cannot achieve. Furthermore,
the conversion efficiency can also be controlled by varying the
intensity of the electric field applied, and it is only the signal
light that is output by turning off the electric field. Moreover,
it was also possible to control the electric field such that the
converted light intensity was maintained at a constant value
against the variable input signal light intensity with fixing the
pumping light intensity and by monitoring the output signal light
intensity.
Although the present embodiment 2 uses the KLTN waveguide, similar
wavelength conversion was implemented by using a KTN waveguide. The
KTN waveguide had a propensity to vary its efficiency more
sensitively to temperature than the KLTN waveguide.
[Embodiment 3]
A device as shown in FIG. 6 was fabricated by adding new electrodes
to the wavelength converter of the foregoing embodiment 2, and an
experiment similar to that of the embodiment 2 was conducted
concerning the wavelength conversion. In FIG. 6, the reference
numeral 19 designates a KLTN waveguide, 20 designates a top
electrode for TM polarization for converting a TM polarization, 21
designates an electrode for TE polarization for converting a TE
polarization, 22 designates an La-doped SrTiO.sub.3 used as a
bottom electrode for the TM polarization and as a substrate. The
wavelength conversion characteristics were measured for both the TE
and TM polarizations this time.
FIGS. 9A and 9B show cross-sections of the electrodes perpendicular
to the waveguide: FIG. 9A shows the placement of the electrode for
the TM polarization; and FIG. 9B shows the placement of the
electrode for the TE polarization. In these figures, the reference
numeral 31a designates an electrode (substrate), 31b designates a
substrate, and 32 designates a waveguide, and reference numerals
33, 33a and 33b each designate an electrode. Since the electric
field distribution differs depending on the electrode structure,
the TE polarization requires about 1.5 times greater electric field
than the TM polarization to obtain the same conversion efficiency
between the TE and TM. However, it was easy to implement
polarization insensitive wavelength conversion by adjusting the
electric field applied. In addition, it was possible to implement
the wavelength conversion of only one of the polarized waves by
turning on and off the electric field.
[Embodiment 4]
A wavelength conversion experiment was conducted using a device
that had nearly the same structure as the device of the foregoing
embodiment 3, and included four types of electrodes with different
element periods provided in the longitudinal direction of the
waveguide. FIG. 7 shows a structure of the wavelength converter
fabricated in this way. In FIG. 7, the reference numeral 23
designates a KLTN waveguide, each reference numeral 24 designates a
top electrode, and 25 designates an La-doped SrTiO.sub.3 used as a
bottom electrode and substrate.
Pumping wavelengths that can achieve phase matching at the element
periods are 0.770, 0.772, 0.774 and 0.776 .mu.m. The wavelength of
the signal light was set at 1.53 .mu.m. The pumping light with the
four wavelengths and the signal light with one wavelength were
launched into the waveguide. Voltages corresponding to one KV/cm
were applied to the four types of electrodes sequentially, and the
converted light was measured by the spectrum analyzer. FIG. 8
illustrates the resultant wavelength conversion spectra. FIG. 8
illustrates that the converted light varies its wavelength
successively in accordance with the changes of the electrodes the
voltages are applied to, and that it functions as a bias in the
wavelength conversion in which the wavelength to be electrically
converted is controlled. In addition, applying voltages to several
types of electrodes makes it possible to convert the signal light
into several wavelengths, which shows that the device is applicable
as a wavelength converter for multicast and the like.
Incidentally, when all the electrodes E1-E4 are turned on, although
the 1552 nm light generated by the conversion by the electrode E1
undergoes further conversion by the electrodes E2 and E3, thereby
generating light with different wavelengths, FIG. 8 illustrates
only the wavelength conversion spectra obtained by eliminating the
light generated by the multiple conversion by filters.
[Embodiment 5]
By using the wavelength converter of the foregoing embodiment 3,
noise figure measurement of the wavelength conversion was conducted
which was carried out by using the signal light (1.543 .mu.m)
modulated to a 160 Gbit/s signal and 0.775 .mu.m pumping light (CW
light). The noise figure, which was measured optically and
electrically, was less than 0.5 dB, with exhibiting no noise
increase by the wavelength conversion. Thus, the present embodiment
5 can respond to a high rate signal to which the wavelength
converter using the optical semiconductor cannot respond. Thus, it
was demonstrated that a noise-free wavelength conversion was
implemented.
[Embodiment 6]
A rectangular waveguide structure was fabricated using the
photolithography and liquid phase epitaxial technique. The
fabricated waveguide has a refractive index difference of 2.5%, and
the cutoff wavelength in a high-order mode is 0.6 .mu.m. Thus, it
functions as a single-mode waveguide for a long wavelength. The
fabricated waveguide was 3 cm long, and the loss of the waveguide
was 0.15 dB/cm. The substrate is composed of SrTiO3 doped with La,
on which gold was deposited to form electrode patterns. It was also
possible to form a similar wavelength converter using KTaO.sub.3 as
the substrate, and deposited Pt as the bottom electrode.
FIG. 7 is a perspective view showing a structure of the wavelength
converter as a wavelength tunable wavelength light source
fabricated in the same manner as the foregoing embodiment 4. The
device was subjected to temperature control by a Peltier device to
stabilize the efficiency and signal wavelength. The periods of the
electrodes that can implement the quasi-phase matching required for
the differential frequency generation of the light with the
wavelength of 1.55 .mu.m band using the pumping light with the
wavelengths of 0.770, 0.775, 0.780, and 0.785 .mu.m. In this case,
the periods of the electrode elements become 12-13 .mu.m. The
output was measured using an optical spectrum analyzer with
applying the voltage of one KV/cm to the electrodes, and launching
the 1.53 .mu.m signal light fed from the signal light generator and
the pumping light with the wavelength of 0.770, 0.775, 0.780, and
0.785 .mu.m fed from the pumping light generator simultaneously
into the incidence end using a polarization-maintaining optical
fiber.
FIG. 8 is a graph illustrating spectra generated by applying the
electric field sequentially to the electrodes as described above.
FIG. 8 clearly shows that the wavelength tunable light source is
implemented by the differential frequency generation.
In addition, the signal light and converted light undergo the
parametric amplification, and the gain of the converted light with
respect to the input signal light reaches about 15 dB, which is a
high gain the conventional LN wavelength converter cannot achieve.
Furthermore, the conversion efficiency can also be controlled by
varying the intensity of the electric field applied, and it is only
the signal light that is output by turning off the electric
field.
It was also possible to control the electric field such that the
converted light intensity was maintained at a constant value
against the variable input signal light intensity with fixing the
pumping light intensity and by monitoring the output signal light
intensity. The wavelength converter could also maintain the
intensity of the output light at nearly the fixed value by
operating it in the gain saturation region. Alternatively, the
output light intensity could be maintained at a fixed value by
applying progressively strong electric field as the electrodes
approached the output side.
FIG. 7 shows a configuration in which the electrode surfaces 24 and
25 are disposed in the vertical direction. However, the TE and TM
polarizations can be generated independently by disposing, besides
the electrodes disposed in the vertical direction, electrodes in
the direction horizontal to the surface, and by supplying the
horizontal electrodes with electric field independently of the
vertical electrodes. FIGS. 9A and 9B are cross-sectional views
showing planar optical waveguide in accordance with the present
invention, which is sectioned normally to the waveguide at the
positions of the electrodes. FIG. 9A shows a structure that
disposes the electrode surfaces 31a and 33 in the vertical
direction, and FIG. 9B shows a structure that disposes the
electrode surfaces 33a and 33b in the horizontal direction.
[Embodiment 7]
In the configuration similar to the embodiment 6, the electric
field modulated by 10 GHz was sequentially applied to the
electrodes. Thus, at 1550, 1560, 1570 and 1580 nm, a light signal
modulated by 10 GHz can be obtained as the need arises. This proves
that it functions as a variable wavelength light source of 10
Gbit/s. The channel spacing can be readily varied by controlling
the periods of the electrode elements, that is, by making phase
matching between the wavelength of the pumping light and the
wavelength of the signal light. Furthermore, if the signal light of
1530 nm consists of a pulse train of 100 GHz of a fiber-ring laser,
the variable wavelength light source can also generate the signal
light of 100 Gbit/s.
Moreover, an increasing number of the electrode patterns fabricated
can easily increase the number of the variable wavelengths. Thus,
the wavelength tunable light source covering a 1250-1700 nm range
was easily implemented by disposing chips with different electrode
patterns in parallel.
[Embodiment 8]
The number of the wavelengths was increased in the same method as
the embodiment 6 except that the present embodiment 8 used the
pumping light with 767.75, 774.75, 784.75 and 804.75 nm, and signal
light including 10 wavelengths of 1528, 1529, 1530, 1531, 1532,
1533, 1534, 1535, 1536 and 1537 nm as the light launched into the
multi-wavelength light source, thereby implementing a multiple
wavelength scheme. FIG. 10 shows the wavelengths obtained by
applying the electric fields to the electrode as required. As shown
in FIG. 10, when the electrodes are each turned on, the signal with
the wavelength corresponding to the differential frequency is
obtained. Accordingly, applying the electric fields to each
electrode makes it possible to double the number wavelengths.
In addition, since turning on all the electrodes will allow each
electrode to generate the differential frequency, and the next
electrode to perform the differential frequency generation again,
the signal light passing through the four stages of the electrodes
will include the total of 160 waves. Thus, applying the present
invention can implement a multi-wavelength light source by a
single-chip device with ease. It is obvious that connecting chips,
each including one electrode, by fibers can also implement a
similar light source. In addition, as clearly seen from FIG. 10,
selecting the electrode to which the electric field is applied
makes it possible to obtain a signal including a necessary number
of wavelengths in a required wavelength range.
[Embodiment 9]
The initially incident 10 wavelengths, which were launched into the
foregoing embodiment 8 of the multi-wavelength light source, were
generated by the multi-wavelength light source with the same
structure as the embodiment 6 including 10 types of electrodes. In
this condition, an experiment similar to that of embodiment 8 was
conducted using a 100 GHz pulse train of a fiber-ring laser or
semiconductor mode-locking laser as the initial signal light among
them. Although all the resultant wavelengths were the same as those
of the foregoing embodiment 8, all the signals were composed of
short pulses modulated by 100 GHz. Thus, the method in accordance
with the present invention has an advantage of being able to
generate the signal light with multiple wavelengths consisting of
short pulses easily.
[Embodiment 10]
The temperature of the waveguide with the electrodes, which was
fabricated by the same method as the foregoing embodiments, was
dropped with applying electric field. The phase transition
temperature of the KLTN crystal constituting the core was 5.degree.
C., and the waveguide was cooled down to -10.degree. C., followed
by removing the application of the electric field. In this
condition, the wavelength conversion was carried out in the same
method as the foregoing embodiments. In this case, although the
electric field was not applied, the foregoing wavelength conversion
efficiency was achieved. This is because the temperature drop
caused the crystal to transition from the cubic system to the
tetragonal system, and the electric field due to spontaneous
polarization generated in the crystal brought about the secondary
nonlinear effect in place of the external electrodes. Using this
method makes it possible to carry out the wavelength conversion
without applying the electric field continuously, and to perform
the same wavelength conversion induced by the electric field as in
the foregoing embodiments by elevating the temperature above the
phase transition temperature. The phase transition temperature of
the crystal can be controlled by varying the composition ratio of
Nb and Ta. Accordingly, the design becomes possible of the
wavelength converter that will minimize the power consumption of
the temperature control near the room temperature by selecting the
composition depending on whether the electric field application
precedes, or the operation is fixed to that does not induce the
electric field.
The present invention has been described in detail with respect to
preferred embodiments, and it will now be apparent from the
foregoing to those skilled in the art that changes and
modifications may be made without departing from the invention in
its broader aspects, and it is the intention, therefore, in the
appended claims to cover all such changes and modifications as fall
within the true spirit of the invention.
* * * * *